PROJECT SUMMARY: The motivation for these studies is the pressing need for a new class of effective and safe antiarrhythmics to prevent VT/VF without compromising EC coupling. CaV channel blockers (Class IV antiarrhythmic agents, such as Diltiazem and Verapamil) have limited therapeutic value because of their negative inotropic effect. These investigators have recently discovered that arrhythmogenic EADs are potently suppressed by the minimal modification of the L-type Ca channel biophysical parameters. Crucially, what these maneuvers have in common is that they all reduce the late component of the L-type Ca current (late ICa,L). The selective reduction of late ICa,L has the benefit of leaving peak ICa,L largely intact, preserving contractility. Regrettably, while the relevance of the ICa,L window current to EAD formation was hypothesized 30 years ago, no therapy based on this idea has emerged yet. The antiarrhythmic strategies that emerged from the investigators? recent studies in animal models of VT/VF are now ready to be pharmacologically implemented: Aim 1 proposes to evaluate LTCC gating modifiers selectively reducing the late component of ICa,L as prototype members of a new class of antiarrhythmics that do not block peak ICa,L. Specifically, Aim 1A will validate the antiarrhythmic potential of pedestal ICa,L reduction using LTCC gating modifiers and determine the efficacy of pilot compounds that selectively decrease late ICa,L: roscovitine enantiomers, known to reduce ICa,L pedestal current. Aim 1B will ?validate the antiarrhythmic potential of ICa,L window current reduction using LTCC gating modifiers?. Gabapentinoids, found in preliminary studies to reduce ICa,L window current by shifting LTCC voltage-dependent activation to depolarized potentials, will be used as pilot compounds. Based on the success of preliminary studies in single cells and full hearts, the ability of these drugs to prevent or suppress VT/VF will be assessed in whole rabbit and rat hearts under EAD-favoring conditions (oxidative stress, hypokalemia). Establishing the VT/VF-preventing efficacy of late ICa,L reduction in two small animal models will justify translation to larger mammalian models and eventually humans. Aim 2, is designed ?to identify the molecular mechanisms underlying late ICa,L reduction by roscovitine and gabapentinoids, prototype members of a potential new class of antiarrhythmic action?. The investigators will take advantage of their recent breakthrough in optically tracking molecular transitions of the human CaV1.2 channel using Voltage Clamp Fluorometry to determine the mechanism by which pilot compounds modify the LTCC to reduce late ICa,L. Specifically, Aim 2A will ?identify the molecular mechanisms underlying pedestal ICa,L reduction by roscovitine? and Aim 2B will ?identify the molecular mechanisms underlying window ICa,L reduction by gabapentinoids?. This information will reveal how next-generation antiarrhythmics should act on the LTCC. Thus, this proposal has two main novel aspects: it sets the basis for a conceptually new class of antiarrhythmics (LTCC gating modifiers) and directs future rational drug design for the LTCC molecule.